The `C_(4)` plants are twice as efficient as `C_(3)` plants in terms of fixing `CO_(2)` but lose only ...... as much water `C_(3)` plants for the same amount of `CO_(2)` fixed.

Explanation

Half, The evolution of the `C_(4)` or plants photosynthetic system is probably one of the stategies for maximising the ability of `CO_(2)` while minimising water loss. `C_(4)` plants are twice as efficient as `C_(3)` plants in terms of fixing carbon in the form of glucose, but lose only half as much water as a `C_(3)` plant for the same amount of `C_(2)` fixed.